Direct observation of ion distributions near electrodes in ionic polymer actuators containing ionic liquids

Effects of dilution in ionic liquid supercapacitors

Dilution in room temperature ionic liquid (RTIL) supercapacitors leads to interesting tricritical phase behavior within the mean-field treatment. The RTIL concentration is a valuable handle for optimizing capacitance and energy storage.

Chemical Vapor Deposition of Ionic Liquids for the Fabrication of Ionogel Films and Patterns

Film deposition and high-resolution patterning of ionic liquids (ILs) remain a challenge, despite a broad range of applications that would benefit from this type of processing. Here, we demonstrate for the first time the chemical vapor deposition (CVD) of ILs. The IL-CVD method is based on the formation of a non-volatile IL through the reaction of two vaporized precursors. Ionogel micropatterns can be easily obtained via the combination of IL-CVD and standard photolithography, and the resulting microdrop arrays can be used as microreactors. The IL-CVD approach will facilitate leveraging the properties of ILs in a range of applications and microfabricated devices.

Low-voltage-driven soft actuators

Soft actuators based on electroactive polymers (EAPs) are the core constituents of future soft robots owing to their fascinating properties such as lightweight, compactness, easy fabrication into various forms, and low cost. Ionic EAP actuators are particularly attractive owing to the low driving voltages (<3 V) as compared to those of electronic EAP actuators (usually kilovolts). This paper presents a brief overview of the recent progress in a range of EAP actuators by focusing on low voltage operation, in addition to the challenges and future strategies for their wide applicability in artificial muscles and various innovative soft robot technologies.

High-Performance Asymmetric Supercapacitors Based on the Surfactant/Ionic Liquid Complex Intercalated Reduced Graphene Oxide Composites

In this paper, ionic surfactants are employed to intercalate thermally-reduced graphene oxide (TRG). The ionic interaction between the intercalated surfactant and the ionic liquid could lead to the formation of large-sized ionic aggregates and, hence, enlarge the interlayer distance between the TRG sheets. The morphology and vibration modes of these composites were systematically characterized using XRD (X-ray diffraction), SAXS (small-angle X-ray scattering), and FTIR (Fourier transform infrared spectroscopy). An asymmetric supercapacitor, which consisted of a cationic surfactant-intercalated electrode on one side and an anionic surfactant-intercalated electrode on the other, was examined. It was found that, with the increased interlayer distance, the energy density and capacitance of the cells were improved. It seems that the cell with a cationic surfactant as the cathode had the best energy density of 67.8 Wh/kg, which is 4.4-fold higher than that of the TRG cell.

XPS enables visualization of electrode potential screening in an ionic liquid medium with temporal- and lateral-resolution

We present an X-ray photoelectron spectroscopic (XPS) investigation of potential screening across two gold electrodes fabricated on a porous polymer surface which is impregnated with the ionic liquid (IL) N-N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide [DEME-TFSI]. The IL provides a sheet of conducting layers to the insulating polymer film, and allows monitoring charging and screening dynamics at the polymer + IL/vacuum interface in a laterally resolved fashion across the electrodes. Time-resolved measurements are also implemented by recording F1s peaks of the IL, while imposing 10 mHz square-wave (SQW) pulses across the two electrodes in a source-drain geometry. Variations in the F1s binding energy reflect directly the transient local electrical potential, and allow us to visualize screening of the otherwise built-in local voltage drop on and across the metal electrodes in the range of millimeters. Accordingly, the device is partitioned into two oppositely polarized regions, each following polarization of one electrode through the IL medium. On the other extreme, upon imposing relatively fast 1 kHz SQW pulses the charge screening is prevented and the device is brought to assume a simple resistor role. A simple equivalent circuit model also reproduces the observed voltage transients qualitatively. The presented structure and variants of XPS measurements, enabling us to record voltage transients in unexpectedly large lateral distances away from the electrodes, can impact the understanding of various electrochemical concepts.

Lattice Model of an Ionic Liquid at an Electrified Interface

We study ionic liquids interacting with electrified interfaces. The ionic fluid is modeled as a Coulomb lattice gas. We compare the ionic density profiles calculated using a popular modified Poisson-Boltzmann equation with the explicit Monte Carlo simulations. The modified Poisson-Boltzmann theory fails to capture the structural features of the double layer and is also unable to correctly predict the ionic density at the electrified interface. The lattice Monte Carlo simulations qualitatively capture the coarse-grained structure of the double layer in the continuum. We propose a convolution relation that semiquantitatively relates the ionic density profiles of a continuum ionic liquid and its lattice counterpart near an electrified interface.

Electrochemical and electromechanical behavior of Nafion-based soft actuators with PPy/CB/MWCNT nanocomposite electrodes

In this work, as an alternative to precious platinum electrodes in IPMC actuators, PPy/CB/MWCNT electrode actuators were successfully fabricated by electropolymerization of PPy on both sides of the CB/MWCNT-coated Nafion membranes.

Improving the Molecular Ion Signal Intensity for In Situ Liquid SIMS Analysis

In situ liquid secondary ion mass spectrometry (SIMS) enabled by system for analysis at the liquid vacuum interface (SALVI) has proven to be a promising new tool to provide molecular information at solid-liquid and liquid-vacuum interfaces. However, the initial data showed that useful signals in positive ion spectra are too weak to be meaningful in most cases. In addition, it is difficult to obtain strong negative molecular ion signals when m/z>200. These two drawbacks have been the biggest obstacle towards practical use of this new analytical approach. In this study, we report that strong and reliable positive and negative molecular signals are achievable after optimizing the SIMS experimental conditions. Four model systems, including a 1,8-diazabicycloundec-7-ene (DBU)-base switchable ionic liquid, a live Shewanella oneidensis biofilm, a hydrated mammalian epithelia cell, and an electrolyte popularly used in Li ion batteries were studied. A signal enhancement of about two orders of magnitude was obtained in comparison with non-optimized conditions. Therefore, molecular ion signal intensity has become very acceptable for use of in situ liquid SIMS to study solid-liquid and liquid-vacuum interfaces. Graphical Abstract ᅟ.

A multiple-shape memory polymer-metal composite actuator capable of programmable control, creating complex 3D motion of bending, twisting, and oscillation

Development of biomimetic actuators has been an essential motivation in the study of smart materials. However, few materials are capable of controlling complex twisting and bending deformations simultaneously or separately using a dynamic control system. Here, we report an ionic polymer-metal composite actuator having multiple-shape memory effect, and is able to perform complex motion by two external inputs, electrical and thermal. Prior to the development of this type of actuator, this capability only could be realized with existing actuator technologies by using multiple actuators or another robotic system. This paper introduces a soft multiple-shape-memory polymer-metal composite (MSMPMC) actuator having multiple degrees-of-freedom that demonstrates high maneuverability when controlled by two external inputs, electrical and thermal. These multiple inputs allow for complex motions that are routine in nature, but that would be otherwise difficult to obtain with a single actuator. To the best of the authors' knowledge, this MSMPMC actuator is the first solitary actuator capable of multiple-input control and the resulting deformability and maneuverability.

Highly Stretchable and Transparent Microfluidic Strain Sensors for Monitoring Human Body Motions

We report a new class of simple microfluidic strain sensors with high stretchability, transparency, sensitivity, and long-term stability with no considerable hysteresis and a fast response to various deformations by combining the merits of microfluidic techniques and ionic liquids. The high optical transparency of the strain sensors was achieved by introducing refractive-index matched ionic liquids into microfluidic networks or channels embedded in an elastomeric matrix. The microfluidic strain sensors offer the outstanding sensor performance under a variety of deformations induced by stretching, bending, pressing, and twisting of the microfluidic strain sensors. The principle of our microfluidic strain sensor is explained by a theoretical model based on the elastic channel deformation. In order to demonstrate its capability of practical usage, the simple-structured microfluidic strain sensors were performed onto a finger, wrist, and arm. The highly stretchable and transparent microfluidic strain sensors were successfully applied as potential platforms for distinctively monitoring a wide range of human body motions in real time. Our novel microfluidic strain sensors show great promise for making future stretchable electronic devices.

Graphitic carbon nitride nanosheet electrode-based high-performance ionic actuator

Ionic actuators have attracted attention due to their remarkably large strain under low-voltage stimulation. Because actuation performance is mainly dominated by the electrochemical and electromechanical processes of the electrode layer, the electrode material and structure are crucial. Here, we report a graphitic carbon nitride nanosheet electrode-based ionic actuator that displays high electrochemical activity and electromechanical conversion abilities, including large specific capacitance (259.4 F g⁻¹) with ionic liquid as the electrolyte, fast actuation response (0.5±0.03% in 300 ms), large electromechanical strain (0.93±0.03%) and high actuation stability (100,000 cycles) under 3 V. The key to the high performance lies in the hierarchical pore structure with dominant size <2 nm, optimal pyridinic nitrogen active sites (6.78%) and effective conductivity (382 S m⁻¹) of the electrode. Our study represents an important step towards artificial muscle technology in which heteroatom modulation in electrodes plays an important role in promoting electrochemical actuation performance.

Electrochemical actuators store electric energy and generate mechanical motion. Here, the authors present an ionic actuator based on a hierarchically porous graphitic carbon nitride electrode, which exhibits high charge storage, fast actuation response, large electromechanical strain and high stability.

Electro-optical phenomena based on ionic liquids in an optofluidic waveguide

An optofluidic waveguide with a simple two-terminal electrode geometry, when filled with an ionic liquid (IL), forms a lateral electric double-layer capacitor under a direct current (DC) electric field, which allows the realization of an extremely high carrier density in the vicinity of the electrode surface and terminals to modulate optical transmission at room temperature under low voltage operation (0 to 4 V). The unique electro-optical phenomenon of ILs was investigated at three wavelengths (663, 1330 and 1530 nm) using two waveguide geometries. Strong electro-optical modulations with different efficiencies were observed at the two near-infrared (NIR) wavelengths, while no detectable modulation was observed at 663 nm. The first waveguide geometry was used to investigate the position-dependent modulation along the waveguide; the strongest modulation was observed in the vicinity of the electrode terminal. The modulation phase is associated with the applied voltage polarity, which increases in the vicinity of the negative electrode and decreases at the positive electrode. The second waveguide geometry was used to improve the modulation efficiency. Meanwhile, the electro-optical modulations of seven ILs were compared at an applied voltage ranging from ±2 V to ±3.5 V. The results reveal that the modulation amplitude and response speed increase with increasing applied voltage, as well as the electrical conductivity of ILs. Despite the fact that the response speed isn't fast due to the high ionic density of ILs, the modulation amplitude can reach up to 6.0 dB when a higher voltage (U = ±3.5 V) is applied for the IL [Emim][BF4]. Finally, the physical explanation of the phenomenon was discussed. The effect of the change in IL structure on the electro-optical phenomena was investigated in another new experiment. The results reveal that the electro-optical phenomenon is probably caused mainly by the change in carrier concentration (ion redistribution near charged electrodes), which induces the enhancement and suppression of NIR optical absorption (contributed by C-H and N-H groups) in the vicinity of the negative electrode and positive electrode, respectively.

Tough nanocomposite ionogel-based actuator exhibits robust performance

Ionogel electrolytes can be fabricated for electrochemical actuators with many desirable advantages, including direct low-voltage control in air, high electrochemical and thermal stability, and complete silence during actuation. However, the demands for active actuators with above features and load-driving ability remain a challenge; much work is necessary to enhance the mechanical strength of electrolyte materials. Herein, we describe a cross-linked supramolecular approach to prepare tough nanocomposite gel electrolytes from HEMA, BMIMBF₄, and TiO₂ via self-initiated UV polymerization. The tough and stable ionogels are emerging to fabricate electric double-layer capacitor-like soft actuators, which can be driven by electrically induced ion migration. The ionogel-based actuator shows a displacement response of 5.6 mm to the driving voltage of 3.5 V. After adding the additional mass weight of the same as the actuator, it still shows a large displacement response of 3.9 mm. Furthermore, the actuator can not only work in harsh temperature environments (100°C and −10°C) but also realize the goal of grabbing an object by adjusting the applied voltage.

Nanothorn electrodes for ionic polymer-metal composite artificial muscles

Ionic polymer-metal composites (IPMCs) have recently received tremendous interest as soft biomimetic actuators and sensors in various bioengineering and human affinity applications, such as artificial muscles and actuators, aquatic propulsors, robotic end-effectors, and active catheters. Main challenges in developing biomimetic actuators are the attainment of high strain and actuation force at low operating voltage. Here we first report a nanostructured electrode surface design for IPMC comprising platinum nanothorn assemblies with multiple sharp tips. The newly developed actuator with the nanostructured electrodes shows a new way to achieve highly enhanced electromechanical performance over existing flat-surfaced electrodes. We demonstrate that the formation and growth of the nanothorn assemblies at the electrode interface lead to a dramatic improvement (3- to 5-fold increase) in both actuation range and blocking force at low driving voltage (1–3 V). These advances are related to the highly capacitive properties of nanothorn assemblies, increasing significantly the charge transport during the actuation process.

Aligned nano-porous microwave exfoliated graphite oxide ionic actuators with high strain and elastic energy density

A high-density aligned nanoporous activated microwave exfoliated graphite oxide (aMEGO) ionic actuator is studied. Before applying an external electric field, the cations and anions are randomly distributed in the composite. After applying the electric field, ions ingress in between the aligned aMEGO sheets through the nanopores to compensate the charges on the electrodes, resulting in the separation of neighboring sheets and unidirectional electro actuation.

Elucidating interactions and conductivity of newly synthesised low bandgap polymer with protic and aprotic ionic liquids

In this paper, we have examined the conductivity and interaction studies of ammonium and imidazolium based ionic liquids (ILs) with the newly synthesised low bandgap polymer (Poly(2-heptadecyl-4-vinylthieno[3,4-d]thiazole) (PHVTT)). Use of low bandgap polymers is the most suitable way to harvest a broader spectrum of solar radiations for solar cells. But, still there is lack of most efficient low bandgap polymer. In order to solve this problem, we have synthesised a new low bandgap polymer and investigated its interaction with the ILs to enhance its conductivity. ILs may undergo almost unlimited structural variations; these structural variations have attracted extensive attention in polymer studies. The aim of present work is to illustrate the state of art progress of implementing the interaction of ILs (protic and aprotic ILs) with newly synthesised low bandgap polymer. In addition to this, our UV-Vis spectroscopy, confocal Raman spectroscopy and FT-IR spectroscopy results have revealed that all studied ILs (tributylmethylammonium methyl sulfate ([N₁₄₄₄][MeSO₄] from ammonium family) and 1-methylimidazolium chloride ([Mim]Cl, and 1-butyl-3-methylimidazolium chloride ([Bmim]Cl from imidazolium family) have potential to interact with polymer. Our semi empirical calculation with help of Hyperchem 7 shows that protic IL ([Mim]Cl) interacts strongly with the low bandgap polymer through the H-bonding. Further, protic ILs shows enhanced conductivity than aprotic ILs in association with low bandgap polymer. This study provides the combined effect of low bandgap polymer and ILs that may generate many theoretical and experimental opportunities.